Method to print photoresist lines with negative sidewalls

ABSTRACT

It is very difficult to produce a negative wall angle from either negative or positive-tone chemically amplified resists, especially by e-beam lithography. This problem has now been overcome by first forming a photoresist pedestal in the conventional way, followed by flood exposing with electrons. Then, a second development treatment is given. This results in removal of additional material from the sidewalls, said removal being greatest at the substrate and least at the pedestal&#39;s top surface, resulting in negatively sloping sidewalls. Application of this method to a process for forming a pole tip for a vertical magnetic writer is also discussed.

This is a divisional application of U.S. patent application Ser. No.10/660,914, filed on Sep. 12, 2003, which is herein incorporated byreference in its entirety, and assigned to a common assignee.

FIELD OF THE INVENTION

The invention relates to the general field of electron lithography withparticular reference to controlling sidewall slope angle.

BACKGROUND OF THE INVENTION

As the critical dimension for devices drops below 100 nm, the wall angle(slope) of photoresist becomes very critical for subsequent etching,deposition, and lift off processes. The resist wall angle may vary frompositive, normal, to negative, as illustrated in FIGS. 1 a, 1 b and 1 crespectively.

Positive and normal sidewall slopes can be easily developed fromnegative-tone chemically amplified resists. However, it is nearlyimpossible to produce a negative wall angle from either negative orpositive-tone chemically amplified resists, especially by e-beamlithography. Due to electron forward scattering in the resist andbackward scattering from the substrate, a positive wall angle is usuallyformed with negative-tone chemically amplified resists. Positive e-beamresists are unable to produce a consistent wall angle and tend toexhibit resist foot necking such as 21 seen in FIG. 2.

As magnetic recording is pushed to higher areal densities, perpendicularrecording has become a serious candidate to replace longitudinalrecording. Perpendicular recording uses a magnetic yoke (surrounded byfield coil) which terminates as a single pole that is used for the writehead. This pole needs to be wide enough at one end to attach to the yokeand narrow enough at its the other end to confine the write flux to avery small area (typically measuring about 0.1 by 0.1 microns). Objectsof this type are most easily formed using micro-molding techniques.Since negative resists can be easily applied to create such molds, it isimportant to be able to control the slope of the sidewalls.

A routine search of the prior art was performed with the followingreferences of interest being found:

U.S. Pat. No. 5,310,626 (Fernandes et al) teaches using a tilt angle inphotolithography while U.S. Pat. No. 6,504,675 (Shukh et al) discloses atrapezoidal write pole. In U.S. Pat. No. 6,255,035, Minter et al.describe two photoresist layers exposed to e-beam to form negativeresist sidewalls and in U.S. Pat. No. 4,238,559, Feng et al. teach thatundercut resist profiles are easily attainable using e-beam lithography.

SUMMARY OF THE INVENTION

It has been an object of at least one embodiment of the presentinvention to provide a method for forming a photoresist pedestal whosesidewalls slope inwards.

Another object of at least one embodiment of the present invention hasbeen for said method to further allow fine tuning of the exact amount ofsaid negative slope.

Still another object of at least one embodiment of the present inventionhas been that said method be suitable for use in electron beamlithography.

A further object of at least one embodiment of the present invention hasbeen to provide a process for manufacturing a trapezoidally shaped poletip for use in a vertical magnetic writer.

These objects have been achieved by first forming a photoresist pedestalin the conventional way. This is followed by flood exposing saidpedestal with electrons followed by a second development treatment whichresults in removal of additional material from the sidewalls, saidremoval being greatest at the substrate and least at the pedestal's topsurface, resulting in negatively sloping sidewalls. Additionally, anapplication of this method to a process for forming a pole tip for avertical magnetic writer is also described.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 a, 1 b, and 1 c illustrate three possible types of sidewall inpedestals formed from photoresist.

FIG. 2 illustrates a particular problem associated with sidewalls whenusing positive e-beam resists.

FIG. 3 shows formation of a latent image in photoresist after exposureto a patterning electron beam.

FIG. 4 shows a photoresist pedestal of the prior art.

FIG. 5 illustrates a key feature of the present invention which is usedto adjust the slope of the sidewalls.

FIG. 6 shows how a pedestal having negatively sloping sidewalls may beformed.

FIG. 7 shows how sidewall slope may be fine tuned.

FIG. 8 is the starting point for a process to form a magnetic writepole.

FIGS. 9-12 show how a suitable mold for said write pole is formed.

FIGS. 13-14 illustrate how the write pole is formed in the mold.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIG. 3, the method of the present invention begins withthe deposition of layer of photoresist 32 on substrate 31. Forphotoresist we have preferred to use 2000$ of NEB22A2 (a negative tonechemically amplified resist from Sumitomo Chemical). The photoresist isthen exposed to pattern generating electron beam 33 (electron dose ofbetween about 10 and 40_C/cm², with about 23_C/cm² being preferred),thereby forming in the photoresist latent image 34 in the shape of arectangular prism. This is followed by a baking treatment (heating to atemperature between about 80 and 120% C for between about 1 and 4minutes, with about 100° C. for 2 minutes being preferred) which in turnis followed by a first development treatment (immersion in a TMAHsolution having a concentration between about 1 and 3% for between about10 and 60 seconds, with about 1.79% TMAH for 13 seconds beingpreferred), selected for the achievement of optimum resolution. Thisresults in the formation of photoresist pedestal 41, as seen in FIG. 4.

Now follows a key feature of the invention. As shown in FIG. 5, pedestal41 is exposed to flooded electron beam 51 a. The latter is generated byraster and/or vector scan. In a first embodiment of the invention,flooded beam 51 a is vertically applied to provide an electron dose ofbetween about 10 and 40_C/cm² with about 26_C/cm² being preferred. Someelectrons, when bombarding the resist, will escape from the resistsidewall but fewer electrons will travel all the way to the bottom.Consequently, more acid will be released from the top and the center ofthe resist pedestal than from the bottom, resulting in a higher degreeof cross-linking closer to the top and further from the bottom.Therefore, the lower sections of the resist pedestal have the potentialto be preferentially dissolved in a suitable solvent.

Accordingly, a second baking treatment (heating to a temperature betweenabout 80 and 120% C for up to 5 minutes, with about 100° C. for 2minutes being preferred) is given, followed by a second developmenttreatment, a key feature being that a more concentrated developer isused this time (immersion in a TMAH solution having a concentrationbetween about 1 and 3% for between about 10 and 60 seconds, with about2.38% TMAH for 60 seconds being preferred), causing sidewalls 61 toslope inwards (at an angle of between about 45 and 90 degrees fromvertical) so that the pedestal is widest at its top surface andnarrowest at the substrate. This is illustrated in FIG. 6.

In a second embodiment of the invention, the flooded electron beam isapplied at an angle relative to the vertical. This is illustrated asbeam 51 b in FIG. 7. Angular exposure of this type leads to a longertraveling path for electrons to escape from the resist sidewall. This,in turn, magnifies the difference in the degree of cross-linking betweenthe top and the bottom sections of the resist pedestal, forming a morenegatively sloping sidewall as illustrated in FIG. 8.

An important advantage of the second embodiment is that it enables theextent of negative sidewall slope to be tunable—the greater the beamtilt, the more negative the sidewall slope.

We have applied the above method to the development of a process formanufacturing a trapezoid-shaped write pole for the use in perpendicularmagnetic recording. This process begins with the formation of aphotoresist pedestal with negatively sloping sidewalls as just describedabove and shown as trapezoidal pedestal 41 in FIG. 8. Typically, thislayer of photoresist is deposited to a thickness between about 0.1 and0.3 microns.

A conformal coating of non-magnetic material 91 (such as aluminum oxideor silicon oxide) is then deposited to a thickness that is sufficient tofully enclose said trapezoidal prism (typically between about 0.1 and0.3 microns) as shown in FIG. 9. The preferred deposition method of thisstep is atomic-layer chemical vapor deposition which is a process ofdepositing successive layers of very thin films, each of which isallowed to react with an appropriate gas before the latter is removedand a fresh thin film layer deposited. The method provides high purityfims having well controlled stoichiometry. This is then planarized untiltop surface 95 of prism 41 is just exposed, as shown in FIG. 10,followed by the removal of all photoresist thereby forming mold 45, asseen in FIG. 11,

All exposed surfaces, including mold 45, are then coated with seed layer92, as seen in FIG. 12. Examples of materials suitable for the seedlayer include, but are not limited to, CoNiFe, NiFe, CoFe, and CoFeN,and it is deposited to a thickness between about 100 and 500 Angstroms.This is followed by deposition, onto the seed layer, of layer 93 of amaterial having high magnetic permeability, to a thickness sufficient tooverfill the mold, as seen in FIG. 13. Suitable materials for layer 93include, but are not limited to, CoNiFe, NiFe, and CoFe, all of themhaving a saturation moment of at least 2.1 Tesla (or 21 kG).

The process concludes with planarizing until seed layer 92 has been justremoved (except inside the mold itself, resulting in the formation ofwrite pole 93.

1. A process to manufacture a write pole for vertical magneticrecording, comprising: depositing, onto a substrate, a layer ofphotoresist and forming therefrom a trapezoidal prism having inwardlysloping sidewalls and a top surface that is parallel to said substrate;depositing a conformal coating of non-magnetic material to a thicknessthat is sufficient to fully enclose said trapezoidal prism; planarizinguntil said top surface is just exposed; then removing all photoresistthereby forming a mold; coating the mold with a seed layer followed bydeposition onto said seed layer of a layer of material having highmagnetic moment to a thickness sufficient to overfill said mold; andthen planarizing until said seed layer is just removed, thereby formingsaid write pole.
 2. The process described in claim 1 wherein saidphotoresist is a negative tone chemically amplified resist.
 3. Theprocess described in claim 1 wherein said layer of photoresist isdeposited to a thickness between about 0.1 and 0.3 microns.
 4. Theprocess described in claim 1 wherein said non-magnetic material isselected from the group consisting of aluminum oxide and silicon oxide.5. The process described in claim 1 wherein said seed layer is selectedfrom the group consisting of CoFe, NiFe, CoNiFe, and CoFeN
 6. Theprocess described in claim 1 wherein said seed layer is deposited to athickness between about 100 and 500 Angstroms.
 7. The process describedin claim 1 wherein said layer of high magnetic moment is selected fromthe group consisting of CoFe, NiFe, CoNiFe, and CoFeX, where X is W, Mn,Ni, Mo, Ti, Nb, V, Cr, or C.
 8. The process described in claim 1 whereinsaid high magnetic moment material has a saturation moment of at leastkilogauss.
 9. A write pole for vertical magnetic recording, comprising:a trapezoidal prism of high magnetic moment material; said trapezoidalprism having parallel top and bottom surfaces and inwardly slopingsidewalls; said parallel surfaces being a distance of between about 0.1and 0.3 microns apart; and said sidewalls having a slope, relative tovertical of up to about 60 degrees.
 10. The write pole described inclaim 9 wherein said high magnetic moment layer is selected from thegroup consisting of CoFe, NiFe, CoNiFe, and CoFeX, where X is W, Mn, Ni,Mo, Ti, Nb, V, Cr, or C.
 11. The write pole described in claim 9 whereinsaid high magnetic moment material has a saturation moment of at least21 kilogauss.
 12. The write pole described in claim 9 wherein the bottomsurface has linear dimensions less than about 0.1 by 0.1 microns.
 13. Aprocess to manufacture a write pole for vertical magnetic recording,comprising: depositing, onto a substrate, a layer of photoresist andforming therefrom a trapezoidal prism having inwardly sloping sidewallsand a top surface that is parallel to said substrate; depositing aconformal coating of non-magnetic material to a thickness that issufficient to fully enclose said trapezoidal prism; planarizing untilsaid top surface is just exposed; then removing all photoresist therebyforming a mold; filling said mold through sputter deposition of a layerof material having high magnetic moment to a thickness sufficient tooverfill said mold; and then planarizing, thereby forming said writepole.
 14. The write pole described in claim 13 wherein said highmagnetic moment layer is selected from the group consisting of CoFe,NiFe, CoNiFe, and CoFeX, where X is W, Mn, Ni, Mo, Ti, Nb, V, Cr, or C.15. The write pole described in claim 13 wherein said high magneticmoment material has a saturation moment of at least 21 kilogauss.